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Sansui 9090DB—amp section works, no sound though

EastPoint

EastPoint

Recycler
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I’m looking at a 9090DB for someone. The driver board and protection board were rebuilt. It comes out of protection fine. With the source on FM I get almost no sound, just a bit in the background. When I touch parts of the preamp section I get buzzing on either channel though. Fuses are good. The tuning meters move but the watt meters do not. What should I be looking at?

I’m assuming some sort of preamp power issue.
 
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A source into the main ins shows that the amp section works great. I have to find some headphones now...
 
That doesn't check the preamp. Put the source into the Aux in. If that works, you have a tuner problem. The tuner power supply is suspect and it also power the Dolby board. So if it goes, no sound output. Measure the DC voltage between pin one and chassis ground. Should be around 12.5V.

- Pete
 
On the tuner power supply board:
Pin 1 13vDC
Pin 2 7.8vDC
Pin 3 53.5vDC

Putting a source into Aux or tape monitor I get a bit of faint music with the volume all the way up, but that's it.
 
Dolby Board pass through joints? Can you poke the joints with a plastic rod? Or at least probe signal going in, but non coming out of board? Seems odd both channels affected the same.
 
I probed around the Dolby board a bunch and didn't get any change. I tried shooting a signal into a few spots and nothing.

The previous tech rebuilt the protection board as well. It is coming out of protection, but maybe something there is wrong, is there a spot I should try shooting a signal?
 
Ok, shooting a signal into the volume pot directly gets me music on either channel.

Shooting a signal into the other end of the tone control board also gets music for both channels. So presumably the tone board is good.
 
It looks like it’s the stupid Dolby board acting up. I found a tutorial online to bypass it fairly easily and now I have sound. I know the Dolby board is a pain in the ass, I just didn’t expect that I would completely lose sound on both channels because of it. I am probably just going to bypass it as I don’t have the time to re-pin the entire thing. Thanks for all the assistance.
 
Here you go:

Dolby board input output.jpg

If you have a strong signal at the input and a weak signal on the output, you have issues with the Dolby circuit. That can be as simple as dirty switches. Clean the bank of Dolby Push switches and the rotary switch before digging deeper.

- Pete
 
On basically every 8080/9090db I've worked on it's not the vias that are the problem on the dolby board but the rotary switch. It needs to be pulled and disassembled to clean it properly. Same goes for selector switch.
 
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I'm all about a bypass, and I go a step further and disconnect it from power. It's not like anything uses Dolby anyhow.

F-2654 Push Switch Circuit Board PCB + FPanel.jpg
 
Even in a working unit, the Dolby function in these receivers no longer serves a practical purpose unless you use it with a tape deck that does not have on-board Dolby functions. These receivers were created at a time when Dolby noise reduction was being introduced so many people had tape decks without Dolby and Dolby FM was an up and coming technology. The industry quickly added Dolby to tape decks and Dolby FM never got off the ground.

This bypass completely removes the Dolby circuit from the signal path while maintaining all tape functions. It no longer goes through any part of the Dolby board including the Dolby rotary switch. In reality the Dolby board can be completely removed. Unfortunately that would leave three vacant holes in the faceplate.

- Pete
 
I'm in the early stages of a 9090db restoration & am still debating the bypass. I get that the dolby is essentially non-essential, but I've got all the time I want on this. Does anyone really regret doing the new bypass that keeps the Tape function intact instead of the board pinning & the switch cleaning to keep it original?
 
PF Sound said:
I'm in the early stages of a 9090db restoration & am still debating the bypass. I get that the dolby is essentially non-essential, but I've got all the time I want on this. Does anyone really regret doing the new bypass that keeps the Tape function intact instead of the board pinning & the switch cleaning to keep it original?
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I rebuilt the dolby board on mine, was sure to solder in pass through pins, replaced all caps and cleaned the dolby switch. 2 years later, still had problems, and jiggling the switch would temporarily help. Last year I did the by-pass. No more problems or any regrets. Debatable if this effects resale, but if the buyer has to jiggle the switch, that would effect resale too.
 
Scott Muni said:
I rebuilt the dolby board on mine, was sure to solder in pass through pins, replaced all caps and cleaned the dolby switch. 2 years later, still had problems, and jiggling the switch would temporarily help. Last year I did the by-pass. No more problems or any regrets. Debatable if this effects resale, but if the buyer has to jiggle the switch, that would effect resale too.
Click to expand...
How exactly did you clean the switch?
 
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